The present invention relates to (a) crosslinked polymers and (b) compositions and methods for (i) crosslinking polymers, (ii) gravel packing well bores, (iii) fracturing subterranean formations, (iv) conforming the penetration depth of a subterranean acidizing procedure, (v) conforming the penetration depth of a subterranean caustic flooding procedure, (vi) inhibiting the migration of a hazardous acid plume in a subterranean stratum, and (vii) reducing the water permeability of subterranean formations to improve the recovery of hydrocarbons.
Polymers are extensively used in well completion and enhanced oil recovery procedures. In many of these procedures, e.g., fracturing, gravel packing, and subterranean formation permeability modification operations, it is preferred to employ crosslinked polymers.
Hydroxyethylcellulose (HEC) is very desirable for use in, inter alia, fracturing and gravel packing operations because the polymer breaks cleanly, i.e., does not cause formation damage or otherwise adversely interfere with the subterranean flow of hydrocarbons. However, heretofore, HEC has been virtually impossible to crosslink. To overcome this problem, U.S. Pat. No. 4,552,215 and U.S. Pat. No. 4,553,601 describe a HEC crosslinking method that requires the additional cost of initially chemically modifying the HEC by incorporating therein a pendant vicinal dihydroxy structure or a pendant aromatic polyol having at least two hydroxyl groups located on adjacent carbon atoms. The modified HEC is then crosslinked using a crosslinking agent selected from titanium (IV), zirconium (IV), antimony (III), antimony (V), boron (III), lead (II), aluminum (III), arsenic (III), chromium (III), and niobium (V), wherein the Roman numerals indicate the respective oxidation state of each of the foregoing ions.